A wireless power transfer system includes a power transmission coil device corresponding to a part of a power transmitter and a power reception coil device corresponding to a part of a power receiver, and implements wireless power transmission using magnetic coupling between coils such as an electromagnetic induction scheme, a magnetic resonance scheme, etc. For example, the wireless power transfer system is applied to a power feeding system of an electric vehicle or a plug-in hybrid vehicle. In this case, the power reception coil device is installed in the vehicle.
Power transmitted between the coil devices changes depending on an inductance of a coil portion. For this reason, an optimum inductance value is determined for each system in terms of transmitted power. However, there is concern that an inductance may change due to a variation of a usage environment such as a temperature, a humidity, etc. or a variation during production. A technology for compensating for the change has been proposed (e.g., see Patent Literature 1). Patent Literature 1 discloses that an inductance is changed by moving a magnetic-field shield such that a positional relationship between a coil and the magnetic-field shield is changed in a power transmitter which includes the coil and the magnetic-field shield corresponding to a magnetic body. Patent Literature 2 discloses that an inductance is changed depending on a distance between a coil and a shield.
Examples of an electric circuit having a coil include a wireless power transfer circuit and a wireless power reception circuit. For example, the wireless power transfer circuit and the wireless power reception circuit are described in Patent Literature 3 below. As in Patent Literature 3, the wireless power transfer circuit includes a power feeding coil for feeding power, and the wireless power reception circuit includes a power reception coil for receiving power wirelessly from the power feeding coil using electromagnetic induction. The wireless power reception circuit supplies the power received by the power reception coil to a battery or a load.
For example, the wireless power transfer circuit and the wireless power reception circuit are provided in an apparatus for feeding power to a vehicle and in the vehicle, respectively. That is, power is fed from the power feeding coil of the wireless power transfer circuit to a battery of the vehicle through the power reception coil of the wireless power reception circuit provided in the vehicle.